The present invention relates generally to frequency generators and more particularly to voltage controlled oscillators used in wireless terminals.
Wireless terminals typically include one or more frequency generators, such as a PLO (Phase Lock Oscillator), a VCO (Voltage Controlled Oscillator), and/or a VCXO (Voltage Controlled Crystal Oscillator), to provide clock signals and a reference frequency necessary for operation of the wireless terminal. A VCXO, which is a crystal-controlled oscillator that varies a frequency in direct proportion to an applied control voltage, typically provides the primary reference frequency for a wireless terminal. VCXOs typically have three inter-related operating parameters: tuning range, DC current consumption, and start-up time. The tuning range represents the difference between the maximum VCXO frequency and the minimum VCXO frequency. A typical tuning range for a VCXO is ±30 ppm. The DC current consumption is the amount of DC current consumed by the VCXO during operation. Typically, a VCXO consumes approximately 10 mA. The start-up time denotes the time required for the VCXO output voltage to reach approximately 90% of the final VCXO output voltage amplitude. Typical worst cast start-up times range from 4 to 5 msec.
Generally, the goal of the VCXO design engineer is to maximize the tuning range while minimizing DC current consumption and start-up time. However, the inter-relation of these three operating parameters makes this goal difficult to achieve. To understand the design trade-offs resulting from the inter-relation of these operating parameters, consider the following. A frequency generated by a VCXO is proportional to a reverse bias voltage applied to a variable load capacitance of the VCXO. The ratio of the minimum reverse bias voltage capacitance to the maximum reverse bias voltage capacitance determines the tuning range. As a result, large capacitance ratios maximize the tuning range.
However, because a large capacitance ratio typically results from a large minimum reverse bias voltage capacitance, and because the start-up time is proportional to the minimum reverse bias voltage capacitance, a large capacitance ratio also results in a large start-up time. Therefore, in order to achieve a desired tuning range, a VCXO may have an undesirably long start-up time.
Conventional VCXO designs may reduce this large start-up time by exploiting the fact that the start-up time decreases with increasing bias current. However, large bias currents result in high DC current consumption. As a result, while an increased bias current may produce a more desirable start-up time, the resulting DC current consumption undesirably reduces the life of the battery associated with the VCXO.